The problem of evil continues to generate a vast literature, but for Aquinas evil is not the insurmountable problem that it is for many modern thinkers. It is simply part of the way the world is. This discussion refers to Summa Theologiae I.49.

Aquinas understands evil in Aristotelian terms of potency and act, cause and effect, lack and perfection, in a teleological context wherein every form of existence has a meaning and purpose particular to its own good. Evil can be explained in metaphysical, natural or moral terms, but it always implies not just the absence of some good but the absence of a good that properly belongs to a species. It is not bad for a human to lack the strength of a lion, for example.

Existence is good, so evil does not exist except as the lack or deprivation of some good that a being should have. We can therefore only recognise evil in the context of a prior understanding of the good of any being. If evil were completely annihilating of the good then it would have to annihilate itself because it depends upon the good of existence to manifest itself as lack.

From a metaphysical perspective, Aquinas asserts that the world is better for having evil within it, because evil serves a greater good. Natural evil contributes to the goodness of creation, and God sometimes inflicts evil as punishment in order to maintain the just order of the universe. Lions kill asses, fire consumes air, humans learn to right wrongs and to endure suffering, and all this is natural and good. However, the projection of morality on to God is a development of modern theism and of the philosophical shift from a naturalistic to a metaphysical account of reason. For Aquinas, God's goodness is beyond all definitions of the good, and we cannot hold God to account by our moral standards. Of course the world could be other than it is, but that would be a different world, and this is the world God created.

From a natural perspective, evil can be unrealised potential, or the unintentional deprivation of some potential that is proper to a species. A baby's inability to walk is bad insofar as she is only potentially a fully developed human, but if she has some disability that prevents her from walking then that is bad because she lacks an ability that is good for the human species. This is different from, say, the potential to become a great musician, which would be a potency and lack only in those humans with an aptitude to become musicians, but not in the generic sense of what is required to actualise one's human potential.

Moral evil results when a person intentionally does something or fails to do something that prevents herself or somebody else from realising their human potential, and it is caused by a defect of the will, that is, by a lack of understanding of the good. Aquinas does not believe that we can rationally desire evil because we can only desire that which exists. Immorality results when we knowingly pursue something that is good in itself but bad for us in terms of what it means to be human. Such acts are liable to punishment by God, and to punishment under the law when they threaten the common good of society.

Some might accuse Aquinas of Panglossian optimism, others of maintaining a punitive or tyrannical view of God. However, let me ask what happens if we apply his understanding of evil as lack to one of the most catastrophic of all moral failures. Could the Holocaust ever be understood only in terms of lack?

The concentration camps were the most extreme example of the wilful deprivation of all the goods humans need to be human: not just the material basics, but dignity, community, love, trust, law and life itself. If we say that this was more than deprivation, that it was caused by the active presence of evil, do we not reduce human responsibility? Aquinas's understanding of morality makes us absolutely responsible for our intentional activities and it allows no excuse in terms of an evil force manipulating our wills. Unless we are suffering from some lack or defect of our natural rational faculties that diminishes our responsibility, we are accountable for what we intentionally do or fail to do. This is still a fundamental criterion of the way the law operates.

When faced with the genocidal horrors of the 20th century, many might find this account of evil as lack deeply problematic. Yet in refusing to situate the power of moral evil anywhere except in the human will, Aquinas's view of morality confronts us with the question that was asked by Rabbi Eliezer Berkovits: not where was God in Auschwitz, but where was man?